The present invention relates to a rolling bearing, and more particularly to a rolling bearing which is effectively used in an environment where the bearing temperature becomes high, such as a rolling bearing for an alternator (an AC generator) in an automobile.
In recent years, in accordance with the requirements for smaller size and reduced weight and for higher output of an automobile, an alternator is used at high-speed rotation with increasing a pulley ratio. In order to prevent slippage of a belt from occurring, the belt tension tends to be increased.
In a rolling bearing used in such an alternator, action of an offset load and vibrations due to high belt tension occur. The rolling bearing is used at high-speed rotation in such a condition, so that the bearing temperature is raised. It is known that, when a rolling bearing made of a steel material, such as a rolling bearing for an alternator, is used at a high temperature for a long time period, the bearing dimensionally expands.
It is known that the dimensional expansion is caused by transformation of austenite remaining in the steel material into martensite. The radial internal clearance of the rolling bearing is reduced by the dimensional expansion. The reduction of the radial internal clearance causes a problem in that, in a bearing used in an environment with relatively large vibrations, e.g., for supporting a rotation shaft of an auxiliary machine for an engine, the reduction may shorten the life period. In the case where a lubricant is deteriorated, particularly a radial internal clearance which is excessively small significantly affects the reduction of the lubrication life period. Thus, it is necessary to avoid the reduction of the radial internal clearance in a long-term use as much as possible.
For such a purpose as described above, and for preventing white layer peeling from occurring, a steel material in which the residual rate of austenite is reduced to be lower than 10% by volume by performing heat resistance treatment in which high-temperature tempering is used in combination with subzero treatment is conventionally employed in a race of a rolling bearing for which high-speed rotation is-required in a high temperature environment, such as a bearing for an alternator (see Japanese Patent Publication (Kokoku) No. HEI7-72565). In this connection, in the case where usual-thermal treatment is performed, the residual austenite rate is 10 to 14% by volume. As described above, by using a race in which the amount of residual austenite is small, it is possible to suppress the dimensional change due to the transformation of austenite into martensite in the use at a high temperature.
When the service environmental temperature is 180xc2x0 C. or higher, however, the expansion of rolling elements, i.e., balls, cannot be disregarded. Therefore, it is necessary to suppress the amount of expansion of the balls by changing thermal treatment conditions, e.g., by raising the tempering temperature. However, as for the production of balls, one lot includes a huge number of balls as compared with a race. When the process is changed only for balls of heat resistant specification, therefore, the cost is increased. Thus, it is difficult to practically adopt such balls of heat resistant specification. In view of the situations described above, a conventional rolling bearing of such a type employs balls which are subjected to standard thermal treatment (tempering at 180xc2x0 C), and hence in which the residual rate of austenite is about 10 to 14%.
In a recent alternator of high-temperature and high speed specification (to be used in such environments where the dmn value is 400,000 or more, the revolution reaches 18,000 rpm, and the temperature exceeds 180xc2x0 C. (which is the standard tempering temperature), however, the dimensional expansion due to the transformation of austenite into martensite in the steel material of the balls cannot be disregarded, as described above. For example, as for balls of model No. 6202, the expansion of about 10 xcexcm occurs for one ball, so that a predetermined clearance (+3 to 18 xcexcm before incorporation, and xe2x88x925 xcexcm to +10 xcexcm as a residual clearance after incorporation, where a tolerance is included) cannot be maintained due to the relative contraction of the race with respect to the balls. Thus, there arises a problem in that clearance loss may be caused and the life period is shortened.
The invention has been conducted in view of the above described circumstances.
It is an object of the invention to provide a rolling bearing such as a rolling bearing for an alternator in which, even when the bearing is used in a high temperature environment, the life period can be prevented from being shortened due to the temporal reduction of a radial internal clearance, without particularly increasing the cost as compared with a conventional rolling bearing of such a type.
In order to attain the above-mentioned object, the rolling bearing of the invention includes a plurality of rolling elements rotatably disposed between an inner race and an outer race, and at least the inner race and the outer race are made of a steel material, an austenite residual rate of the inner race is smaller than an austenite residual rate of the outer race.
In the invention, a configuration in which the austenite residual rate of the outer race is 10% by volume or more, and the austenite residual rate of the inner race is 6% by volume or less can be preferably adopted. A more preferred range of the austenite residual rate of the inner race is 3 and by volume or less. In the invention, in order to realize the configurations of the austenite residual rates of the inner race and the outer race described herein, a configuration in which, among the inner race, the outer race, and the rolling elements, heat resistance treatment is performed only on the inner race may be adopted.
As the heat resistance treatment performed for the inner race, treatment including high-temperature tempering at 200 to 270xc2x0 C. or higher may be used.
The invention is intended to attain the above mentioned object by setting the amount of dimensional expansion of the outer race to be larger than the amount of dimensional expansion, of the inner race by anticipating dimensional expansion of each member due to the martensite transformation of residual austenite in the steel material.
Specifically, when the residual austenite rate of the outer race is set to be larger than the austenite residual rate of the inner race, amounts of dimensional expansion of respective members caused by transformation of residual austenite into martensite because of the use in a high temperature environment of the outer race are larger than those of the inner race. As a result, it is possible to suppress the reduction of the radial internal clearance due to the dimensional expansion.
In one embodiment of the invention, the austenite residual rate of the outer race is 10% or more and the austenite residual rate of the inner race is 6% by volume or less. When, for example, a rolling bearing having a size similar to that of model No. 6202 is used for a long time period in an environment exceeding 180xc2x0 C., and when a steel material which is subjected to usual treatment is used for rolling elements, an amount of dimensional expansion caused by the transformation of residual austenite into martensite is about 10 xcexcm, and an amount of dimensional expansion of the outer race is about 20 to 30 xcexcm, but a small dimensional change occurs in the inner race. Even if the rolling bearing is used for a long time period in such an environment, therefore, the occurrence of clearance loss can be effectively prevented from occurring.
In another embodiment of the invention, the austenite residual rate of the inner race is smaller than that of the outer race and this can be attained by performing heat resistance treatment only on the inner race among the inner race, the outer race, and the rolling elements. Accordingly, the above-mentioned functions and effects of the invention can be attained by performing usual heat treatment on the outer race and the rolling elements. Thus, the production cost will not be increased as compared with a conventional rolling bearing for an alternator or the like.
As the heat resistance treatment for setting the austenite residual rate of the inner race to be 6% by volume or less, and more preferably 3 and by volume or less, treatment including high-temperature tempering treatment at 200 to 270xc2x0 C. or higher, or, in addition to the treatment, subzero treatment is further performed, is suitable.